Generally, the two common forms of fluorescent lamps are the so-called "preheat" type or the "rapid-start" type. The preheat type of fluorescent lamp has heater current flow through each electrode during lamp ignition and thereafter a voltage-sensitive starter, located external to the lamp or within the lamp itself, opens and discontinues the above-mentioned heater current flow. However, the "rapid-start" type of fluorescent lamp normally has current flow through each electrode not only during ignition but also during the operational period of the lamp. Thus, it can readily be seen that this continuous flow of heater current during operation of the rapid-start lamp is a cause for power loss in the system and an obvious and undesired cause for reduced operational efficiency.
In an effort to improve the energy efficiency of rapid-start fluorescent lamps, numerous suggestions and structural configurations have been suggested. For example, U.S. Pat. Nos. 4,052,687; 4,097,779; 4,114,968; 4,156,831; and 4,171,519 all of which are assigned to the assignee of the present application, provide numerous configurations for enhancing the operation of rapid-start fluorescent lamps. Primarily, each of the above-listed patents relates to rapid-start fluorescent lamps or bimetal type circuit breakers for fluorescent lamps whereby heater current flow is discontinued during lamp operation.
Although the configurations and techniques disclosed by the above-listed patents enhance and provide numerous advantages over prior known configurations and techniques, it has been found that problems still remain. The electrodes of the fluorescent lamp include a carbonate material which requires activation during lamp manufacture by passing current through the electrodes. During this manufacturing step, the circuit breaker is subjected to temperatures sufficiently high (about 300 degrees Celsius) which otherwise maintain the circuit breaker open. Therefore, it has been found necessary to provide a means for essentially removing the circuit breaker effect from the mount structure until after the carbonate material of the electrode has been activated.
In order to accomplish the activation of the carbonate material, an electrical shunt material is coupled across the leads of the circuit breaker to permit current flow to the electrodes. However, once the carbonate material on the electrodes has been processed, it is necessary to remove the electrical shunt material by applying an electrical pulse through each of the shunts. Although such structures have been and still are employed with relatively good results, it has been found that a circuit breaker wherein such electrical shunts are required does present problems of extra materials, added labor, increased defects and reduced productivity all of which increases cost and reduces manufacturing efficiency. Moreover, lamp processing introduces numerous oxidation problems associated with correct attaching of the shunt material.
U.S. Pat. No. 4,517,493 is an example of a fluorescent lamp having a circuit breaker which does not require the above-described electrical shunt across the circuit breaker. This patent teaches a third lead-in wire (19) in each mount which bypasses the circuit breaker and allows the cathodes to be activated. After each lamp cathode has been activated, the third lead-in wire is not further used. An additional step is required to clip off or bend out of the way the end portion of this lead-in wire presumably so as not to interfere with the remaining lead-in wires (16, 17).
Other attempts have been made to provide a circuit breaker which does not require the above-described electrical shunt or a third lead-in wire. For example, U.S. Pat. Nos. 4,600,861, 4,616,156 and 4,659,966 all of which are assigned to the assignee of the present application, describe tri-model circuit breakers. While these circuit breakers may provide relatively good results, they may require some precision in forming since each closed position requires the bimetal to engage two separate contact points within the circuit breaker. The two separate contact points may undesirably exhibit different contact resistances.